Human type 1 diabetes (T1D) is characterized by the specific immune-mediated destruction of insulin- producing pancreatic beta cells. Importantly, earlier studies demonstrated that immune memory to beta cell antigens plays a significant role in their destruction, which constitutes a major obstacle for long-term acceptance of islet cell grafts (1-3). There is now consensus that CD8 T cells constitute the principal T cell type in insulitis in recent-onset patients (4). Such CD8 T cells are potentially very harmful, since they have been shown to readily kill human beta cells in vivo, if enough MHC class I is upregulated on their surface (5). However, not much is known about the overall CD8 specificities and frequencies present in islets and the cause for their entry and activation. Possible targets are known autoantigens derived from beta cells such as insulin, IGRP, IA-2 and GAD (see Table 1 for abbreviations) and cellular matrix proteins, which could become presented when beta cells are destroyed but also viral proteins for example enteroviral determinants. The overall objective of this proposal is therefore to reveal the specificity of CD8 T lymphocytes that are found in human islets and address whether viral infections may play a role in this scenario. [In parallel studies, we aim to confirm our preliminary data showing that islet infiltration may persist long beyond clinical diagnosis and in such cases follows a continuous course both quantitatively and qualitatively. The latter finding would have broad implications on the feasibility of experimental tolerization therapies in longstanding patients.] Our first goal is to systematically detect autoreactive CD8 T cells within human islets and correlate their numbers and activation status with the local histopathology of the islet. In situ tetramer staining of freshly frozen human pancreata and pancreatic lymph nodes now available through the unique nPOD organ repository will be used. In addition, humanized HLA A2 expressing transgenic mice will be utilized to map responses to novel cellular matrix (self) epitopes which will then be validated on human tissues. Our second goal is to search for [CVB]-specific T cells within islets and address the important question, whether viral infection makes islets more accessible for autoreactive T cell or vice versa. To this purpose human sections from pancreata of diabetic patients will be probed with [CVB-specific] tetramers and viral nucleic acids will be detected in collaboration with Heikki Hyoty and Stephen Tracy. In addition, diabetes- prone HLA-A2 humanized mice and fluorescently labeled Coxsackie B virus strains will be used to address the fundamental 'chicken-egg' question, whether viral infections are more important to condition islets for autoreactive lymphocyte entry and destruction, or, conversely, whether autoreactive attacks make islets more accessible for enteroviral infections.